Uranus and Neptune are considered sister
planets. We call them sisters because they are so similar.
Pluto, however, is different from every other planet in the Solar
System. Pluto is actually more like a comet than a planet.

Uranus and Neptune:

We began our journey through the Solar System
talking about the small dense inner planets. We then explored
the very large middle two planets Jupiter and Saturn. Uranus
and Neptune are in between. They are not gigantic, but they are
not small either. Uranus takes up 67 times as much space as the
Earth while Neptune takes up 57 times as much space.

Planetary ingredients:

In addition to being so similar in size, these two
worlds are also made up of the same ingredients. They have a
hard rock core about the size of the Earth. Around that rock
core is a huge liquid water ocean thousands of miles deep.
Think about that, these two worlds actually have liquid water
oceans. On top of the water oceans sits another ocean of liquid
metallic hydrogen.

Both Uranus and Neptune have an atmosphere of
mostly hydrogen, helium and methane. The weather of these
atmospheres is quite active. One very large storm on Neptune is
similar to Jupiter's great red spot. This storm is called The
Great Dark Spot.

Life:

After talking about the gigantic water oceans of
these two worlds, we need to talk about life. Wherever we find
liquid water, we believe there is a possibility we might also find
life. This may or may not be true for Uranus and Neptune.

The first problem that life would encounter is that
at the enormous depths of this ocean there would be absolutely no sunlight whatsoever. Life on the Earth requires either sunlight
or heat from volcanoes to absorb energy.

The second problem that would make it difficult for
there to be any life on Uranus or Neptune is the titanic pressure
that would squeeze any plants or animals very tightly. It is
impossible for humans to visit the depths of our own oceans, even in
submarines because of the pressure. The pressure of these
planets' oceans would be millions of times more powerful. All of
this does not mean there is not life on Uranus or Neptune. It
just means that it is unlikely.

Rings:

Saturn's beautiful ring system is not alone in the
Solar System. Along with Jupiter, Uranus and Neptune both have
a system of rings.

Differences:

There are not many differences between these two
worlds, but there are some. The major difference is the way
these planets rotate. Every planet in the Solar System rotates
counter clockwise except for Uranus. Uranus rotates, or spins, on
its side.

It is believed that billions of years ago a very
large object, possibly as large as the Earth, smashed into the side of
Uranus. This collision was so powerful that it completely
changed the rotation of Uranus. Another new idea about why Uranus spins on its side is that long ago when our Solar System was still new, perhaps a large moon that orbited Uranus was pulled away by another very large planet. It is thought that maybe the force of this large moon being pulled away caused Uranus' axis to tilt, leaving Uranus to rotate on its side.